The goal of this research is an understanding of the action of near-ultraviolet radiation (near UV; 300-380 nm) on living cells. This radiation is present in sunlight and has considerable ecological and medical importance. It is to be distinguished from far-ultraviolet (germicidal) radiation (far UV; below 300 nm), which is highly absorbed by proteins and nucleic acids and is not significantly present in sunlight. There are three long-term objectives of this research: (1) elucidation of the molecular mechanisms of near-UV actions, in particular that of inhibition of cell growth, (2) insight into the ecological and medical importance of these mechanisms, as indicated by the extent of their occurrence in living systems and the ease with which they are induced by sunlight, and (3) the use of these findings as tools in biological research, a possibility wherever the effect is found to be highly specific at the molecular level. An example of a near-UV effect which may well contribute to all of these objectives is that of near-UV-induced growth delay, whose mechanism our research now indicates is a specific action on transfer RNA, whose ecological significance we have demonstrated by sunlight experiments, and which, because of its apparently high specificity, may well prove to be useful probe of protein synthesis. We propose to continue our studies into the molecular mechanisms of near-UV-induced growth delay in bacteria. This work will also involve some studies of photoprotection, which is related to growth delay. We also plan to do some work on the lethal effects of near-UV radiation, particularly studies of chromophore and target molecules. Some studies on the effects of near-UV radiation on bacterial membranes (oxidative phosphorylation and active transport) will also be done.